Beta2-glycoprotein I (β2GPI) is the major target for autoimmune antibodies associated with antiphospholipid syndrome (APS), an autoimmune disease characterized clinically by clots (thrombosis) in both arteries and veins, as well as pregnancy-related complications such as miscarriage, stillbirth, preterm delivery, and preeclampsia (Atsumi et al., Autoimmunity 38:377-381, 2005; Galli et al., Lancet 335:1544-1547, 1990; McNeil et al., Proc Nat Acad Sci USA 87:4120-4124, 1990; Miyakis et al., J Thromb Haemost 4:295-306, 2006). Presently, APS patients with thrombotic complications who have high titers of antibodies are treated chronically with anticoagulants (George et al., Prog Cardiovasc Dis 52:115-125, 2009; Lim et al., JAMA 295:1050-1057, 2006; McKnall-Knapp, Curr Rheumatol Rep 10:62-66, 2008). However, even continuous anticoagulation may not prevent recurrent thrombosis (George et al., Prog Cardiovasc Dis 52:115-125, 2009), emphasizing the need for a more effective antithrombotic therapy based on the thrombogenic mechanisms specific to APS.
β2GPI consists of five domains connected by short three- to four-residue linkers (Bouma et al., Embo J 18:5166-5174, 1999; Schwarzenbacher et al., Embo J 18:5166-5174, 1999). Flexible linkers between domains permit β2GPI to adopt different overall shapes such as a fishhook-like shape seen in the crystal structure (Bouma et al., Embo J 18:5166-5174, 1999; Schwarzenbacher et al., Embo J 18:5166-5174, 1999), an S-shape observed by small angle x-ray scattering for β2GPI in solution (Hammel et al., J Mol Biol 321:85-97, 2002) and a circular shape detected by electron microscopy (Agar et al., Blood, 2010). The circular shape in which domain I is adjacent to domain V is the predominant conformation of β2GPI in normal human plasma (Agar et al., Blood, 2010). Circular β2GPI can be converted to an extended form by altering pH and salt concentrations, binding to a high-affinity antibody directed to domain I or by the binding to cardiolipin (Agar et al., Blood, 2010). β2GPI, which is abundant in plasma (about 170 μg/ml) (Lin et al. Lupus 15:87-93, 2006), acquires its prothrombotic properties only in the presence of anti-β2GPI antibodies. Antibodies of the IgG isotype have the highest correlation with the clinical manifestations of APS compared to other identified antibodies (Laat et al., Blood 104:3598-3602, 2004; Guerin et al., Autoimmunity 31:109-116, 1999). Although anti-β2GPI antibodies in APS patients are highly heterogeneous in respect to their affinity for β2GPI and the location of their binding epitopes, autoantibodies against domain I are the most common and better correlate with thrombosis (Laat et al., J Thromb Haemost, 2009; Ioannou et al., J Thromb Haemost 7:833-842, 2009). The presence of anti-β2GPI antibodies causes cellular activation both in vitro and in vivo (Cugno et al., J Autoimmun 34:105-110, 2010; Koike et al., J Autoimmun 28:129-133, 2007; Pierangeli et al., Thromb Res 114:467-476, 2004). Toll-like receptors, annexin A2, ApoER2, GPIb and anionic phospholipids exposed on cellular surfaces are suggested to be pathologically important in APS (Koike et al., J Autoimmun 28:129-133, 2007; Pierangeli et al., Thromb Res 114:467-476, 2004; Lutters et al. J Biol Chem 278:33831-33838, 2003; Pennings et al., J Thromb Haemost 5:369-377, 2007; Shi et al. Arthritis Rheum 54:2558-2567, 2006; Cockrell et al., Lupus 17:943-951, 2008; Raschi et al., Lupus 17:937-942, 2008; Urbanus et al., J Thromb Haemost, 2008; Pierangeli et al., Ann Rheum Dis 66:1327-1333, 2007; Romay-Penabad et al., Blood 114:3074-3083, 2009; Urbanus et al., Blood Rev 22:93-105, 2008; Rand et al., Lupus 19:460-469, 2010). The binding sites for anionic phospholipids (Hunt et al., J Immunol 152:653-659, 1994; Mehdi et al., Eur J Biochem 267:1770-1776, 2000; Sanghera et al., Hum Mol Genet 6:311-316, 1997; Sheng et al., J Immunol 157:3744-3751, 1996), lipoprotein receptors (Lummel et al., J Biol Chem 280:36729-36736, 2005), and GPIb (Pennings et al., J Thromb Haemost 5:369-377, 2007) are in domain V of β2GPI (β2GPI-DV).
At present, the standard of care for APS is treatment with aspirin to inhibit platelet activation and/or warfarin as an anticoagulant. The goal of the prophylactic treatment is to manage the patient's clotting profiles (e.g., the patient's prothrombin time (PT)), as determined by, e.g., the prothrombin ratio (PR) or international normalised ratio (INR). Treatment is usually given only to patients that have experienced thrombotic symptoms. During pregnancy, low molecular weight heparin and low-dose aspirin are used instead of warfarin because of warfarin's teratogenicity. APS continues to be a health issue, especially for women, who constitute 75-90% of those affected by APS. There is a need for improved therapies for the treatment of APS in patients.